In general, an engine sucks, compresses and explodes a mixture of fuel and oxygen. Exhaust gas, which is created when the mixture of fuel and air is exploded, is discharged into the atmosphere through an exhaust pipe, and a large amount of materials such as carbon monoxide (CO), nitrogen oxide (NOx), and unburned hydrocarbon (HC), which are harmful to the human body, is included in the exhaust gas.
In particular, in comparison with a gasoline engine that uses gasoline and an LPG engine that uses LPG, in the case of a diesel engine that uses diesel, a large amount of nitrogen oxide (NOx), which is a harmful material, is produced because fuel is combusted in an environment in which oxygen is excessive, and it is difficult to remove nitrogen oxide because fuel is combusted in a lean-burn environment.
Therefore, an exhaust gas post-processing apparatus is mounted in a diesel vehicle in order to reduce the aforementioned harmful material, and a urea-based selective catalytic reduction apparatus is representatively used.
The UREA-SCR system uses a method of supplying urea in an aqueous solution form to an exhaust gas line, allowing nitrogen oxide (NOx) and urea (NH2—CO—NH2), which is obtained by thermally decomposing urea using high-temperature exhaust gas, to react, and converting nitrogen oxide (NOx) into nitrogen (N2) and oxygen (O2).
However, in the selective catalytic reduction apparatus, high-temperature exhaust gas discharged from an engine flows into a selective catalytic reduction (SCR) via a diesel oxidation catalyst (DOC) and a diesel particulate filter (DPF), and as a result, there is a problem in that there is a time delay when a temperature of a front end of the selective catalytic reduction and a temperature of a carrier is raised to an appropriate temperature because heat is emitted while the exhaust gas is moved.
In a case in which temperatures of the selective catalytic reduction apparatus and the front end of the selective catalytic reduction are low when the engine initially starts as described above, a crystallization phenomenon in which urea is accumulated in a pipe when a urea aqueous solution is injected (a phenomenon in which the urea is chemically converted into ammonia (NH3) while being thermally decomposed at a temperature of about 100° to 150° or more after the urea is injected, and the urea is whitely accumulated while being attached to an inside of the pipe merely in the urea state at a low temperature), such that the system is destroyed, or an amount of urea, which flows into a carrier of the selective catalytic reduction, is smaller than an amount of produced nitrogen oxide (NOx) due to a partial shut-off phenomenon, and as a result, there is a problem in that a reducing rate of nitrogen oxide (NOx) becomes low.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.